Sandwich assays generally proceed by adsorbing a target analyte onto a surface coated with a capture agent. The target analyte is then detected using a detection agent that also binds to the target analyte at a different site than the capture agent. Signal from the detection agent is used to detect the target analyte. For example, a substrate may include a number of capture agents on its surface. A fluid sample including detection agents and target analyte are introduced to the surface. The target analyte binds to the capture agent. The detection agent also binds to the target analyte. In this manner, complexes including a capture agent, a target analyte, and a capture agent may be formed on the substrate. Some free detection agent may remain in the fluid sample and is not involved in a complex. The free detection agent is not representative of the presence of target analyte, because it is not bound to the target analyte. That is, the unbound detection agent may generate a false positive signal indicating the presence of the target analyte. Accordingly, the signal from the free detection agent may obscure accurate detection. Accordingly, multiple wash steps are performed to rinse away the free detection agent, leaving only complexed detection agents bound to a target analyte remaining on the substrate.
The detectable signal from the detection agent bound to the substrate, however, may be too low for accurate detection. For example, the complexed detection agent may be spread across too large an area of the substrate to generate sufficient signal for detection. Accordingly, additional labeling agents may be added and may bind to the complexes to increase the amount of signal generated by the complexes.
In the case of a target analyte such as a bacterial pathogen or other nucleic acid analyte, the detection process can take several days and require a highly trained specialist to examine the morphology and phenotype of the bacteria. In addition, although molecular biology techniques such as Southern blots, Western blots, and PCR have been adapted for clinical use, these techniques require amplification of the signal through thermocycling and secondary antibodies, thereby causing further delay.